Osprey Sirrus

Consider your activity

• Hiking: Often compatible with hydration reservoirs, have external gear straps/loops
• Running: Designed to limit bounce, hydration compatible
• Climbing: Slim profile, abrasion-resistant reinforcements
• Cycling: Road cycling packs compact with lower profile; MTB packs hold more gear

How much gear will you carry?

• 10L or less: Short runs, quick outings, casual use, essentials only
• 11–20L: Day hikes, cycling, can carry an extra layer
• 21–35L: Sweet spot for multi-hour or full-day hikes, travel
• 36–50L: Gear-heavy outings l(all-day hikes, climbing), winter hiking, ultralight backpacking

Internal frame or frameless?

• Internal frame: More support for heavier loads, but bulkier
• Frameless: Lighter weight and more compact, but less support

Find the right fit

• Use torso length (not overall height) to find your pack size

• Measure your hip circumference (slightly higher than your pant-waist size) and check that against the hipbelt size

Choose a gear capacity 

• Runs under 2 hours: 2 liters 
• Runs 2-3 hours: 2 to 6 liters 
• Runs 3-6 hours: 4 to 12 liters 
• Runs over 6 hours: 6+ liters 

Decide on bottles and/or reservoir 

• Most vests are compatible with both bottles and reservoirs. 
• Bottles are easier and faster to refill, usually holding 500 ml of water each. 
• Bottles can be soft or rigid. Rigid bottles are more durable.  
• Reservoirs hold more water, up to several liters. 

Get the right fit 

• Follow the manufacturer’s sizing guidelines. 
• Choose a snug fit. (Measure your chest while wearing your sports bra.) 
• Leave room for adjustment: The straps should sit in the middle of their adjustment range. 
• Women-specific vests offer fits optimized for the bust and shoulders. 

Capacity

Trip duration is the biggest factor. General guidance:

• Weekend (1-3 nights): 30-50 liters
• Multiday (3-5 nights): 50-80 liters
• Extended (5+ nights): 70-plus liters

Features
Consider frame type, storage, padding and ventilation.

Fit
Sizing is based on torso length (not height) and hip circumference. Sizing can vary by brand and model, so check a pack's size chart.

Finding torso length and hip circumference

• Locate your C7 vertebra by tilting your head forward and feeling for the bony bump between your shoulder blades.
• Place your hands over your hip bones. Draw an imaginary line connecting your thumbs across your lower back.
• Stand up straight. Have a friend measure from your C7 vertebra to the imaginary line between your thumbs. This is your torso length.
• To find hip size, measure around the top of your hips, using the same imaginary line you found earlier.